1. Field of the Invention
The present invention relates to a receiver, and more particularly to a receiver capable of demodulating a received signal with optimum power consumption and optimum reception performance.
2. Description of the Related Art
Development of a received-signal processing method capable of providing maximum reception performance with minimum power consumption has been lagging. Conventional receivers typically use a delay detection scheme or a synchronous detection scheme for the purpose of processing a received signal when a n/4-shift QPSK (Quadrature Phase Shift Keying) signal or a QAM (Quadrature Amplitude Modulation) signal is received. The difference between the two detection schemes is shown in Table 1.
TABLE 1DetectionSchemeCharacteristicsExperienced ProblemsDelaySimple circuitPerformance when the receivedDetectionconstructionsignal has a large delay dispersionSmall poweris lower than when synchronousconsumptiondetection is utilized.SynchronousComplicatedPoor reception performancedetectioncircuitwherein it is difficult to maintainconstructionsynchronization (e.g., when theLarge powerS/N rate of a received signal isconsumptionlow or when paging is large)
That is, receivers using the delay detection scheme and the synchronous detection scheme have the problems described in Table 1. Receivers which select a detection scheme according to reception states of received signals have been proposed to solve the aforementioned problems as described in Table 2 (e.g., Japanese Laid-Open Patent Publication Nos. P08-23232 (“The 233 and 072 Documents”) and P07-23072 and Japanese Patent Registration Publication No. 2977396 (“The 396 Document”)).
TABLE 2Prior artCharacteristicsProblemTheOn the basis of the amount ofTo obtain superior396intersymbol interference or anreception performanceDocumenterror in signal judgment,to that required by theconversion is performed to useuser, more power than isone detection scheme having anecessary may bebetter characteristic and toconsumed.operate any one of two-sidecircuits, thereby achieving a lowpower consumption rate.The 232 andOn the basis of the level of aSame as above072received signal, Conversion isDocumentsperformed to use one detectionscheme having a bettercharacteristic.
In addition, receivers for a CDMA (Code Division Multiple Access) signal or a spread spectrum signal have been proposed. Such receivers (i.e., RAKE receivers each of which contains a plurality of fingers) changes the number of fingers which operate according to reception states of received signals, thereby achieving low power consumption (e.g., Japanese Laid-Open Patent Publication Nos. P11-261523 and 2000-252896 (“the 523 and 896 Documents”).
Although the aforesaid receivers (i.e., those receivers as described in the '232, '072, '396, 523, and '896 documents, and the like) would benefit users if the above-mentioned receivers provide increased reception performance, they also suffer from increased power consumption. To be specific, in the case of the receivers described in the 232 and 072 documents, since two circuit sections corresponding to the two detection schemes simultaneously operate, the amount of power consumption must greatly increase in order to obtain the best reception performance. In this case, the receivers can be used without any problem if a user wants stable processing of a received signal through the best reception performance, but shortening the lifetime of a battery (e.g., battery duration) by simultaneously operating the two circuit sections corresponding to the two detection schemes, is highly undesirable.
Meanwhile, the 396 document, in which power consumption is considered, discloses a technique for continuously monitoring the reception state of a received signal by the amount of intersymbol interference or by an error in signal judgment for the purpose of determining one of a delay detection circuit and a sub-synchronous detection circuit to be used.
However, in the case of the technique, when a user does not want the stable processing of a received signal through the best reception performance but wants to extend battery lifetime (i.e., increase the duration of the battery between charges), the operation of monitoring a reception state of a received signal is unnecessary, so that unnecessary power consumption caused by the unnecessary monitoring operation is against the benefit of the user.
Also, according techniques disclosed in the 523 and 896 documents, a RAKE receiver containing a plurality of fingers changes the number of fingers to operate according to reception states of received signals, thereby achieving low power consumption. However, in a RAKE receiver according to this technique, when more fingers operate than those necessary for the reception quality required by a user, unnecessary power is consumed and is undesirable.